Additive manufacturing or rapid prototyping methods for producing objects comprise layer-by-layer solidification of a material, such as a metal powder material, using a laser beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. In a single build, more than one object can be built, the objects spaced apart in the powder bed.
It is known from DE102005014483 A1 to use four laser beams, each laser beam solidifying powder in a different quadrant of the powder bed. Such an arrangement may increase build speed because different parts of an object or different objects located in different quadrants can be built simultaneously with different laser beams. However, the lasers may be underutilised during the build if, for any one of the lasers, the area to be solidified is larger in one of the quadrants than in the others. For such a layer, the lasers of the other quadrants will be off whilst the laser for the quadrant comprising the largest area to be solidified completes solidification of that area. Therefore, there is a limit on the speed of the build set by the time it takes the laser of the quadrant with the largest area to be solidified. As the laser modules are a very expensive part of the apparatus, significantly increasing the cost of the apparatus by increasing the number of lasers but, at the same time, not using some of the lasers for large durations of the build is undesirable.
US2013/0112672 A1 discloses an additive manufacturing assembly for producing a plurality of laser beams for melting layers of material in an additive manufacturing process. Each laser beam is separately and independently directed to different regions within the workspace. Each region comprises overlapping areas within adjacent regions. The overlapping extension of each of the laser beams provides a consistent melting of powdered metal at the boundaries separating the regions. The overlapping portions and melting provided by adjacent beams in adjacent regions prevents undesired incomplete melting, or possible knit lines, within the completed part. In other words, each laser beam is capable of being directed to the overlapping region such that the part fabricated will include a complete melting and coverage of the metal powder during formation of the part. Like DE102005014483 A1, there is a limit on the speed of the build set by the time it takes the laser with the largest area of powder to be solidified to solidify this area. During this time, the other lasers will be underutilised.
JP2009006509 discloses a method of manufacturing a three-dimensional article with a plurality of laser beams. Each optical beam can be scanned by a dedicated module across the entire powder bed of the build area, with an area of powder to be solidified in the powder bed assigned to the lasers such that area to be scanned by each laser for each layer is equal. JP2002144437 and JP2000263650 disclose similar arrangements. U.S. Pat. No. 5,536,467 discloses apparatus for producing a three-dimensional object, wherein multiple laser beams are used to cure light curable resin. Each laser beam may be directed to predetermined regions of a layer to solidify those regions.
In all these arrangements, the fixed optical modules for scanning the laser beams must be spaced apart such that, for each module to be capable of directing the corresponding laser beam to any location in the powder bed, each optical module must be configured differently based on its location. This may require a sub-optimal arrangement of the optical module and/or non-utilisation of the full range of the optical module across which the module provides an acceptable performance.
DE19953000 discloses a device for rapid production of bodies by selective sintering of powder layers. The device uses two laser beams that may be delivered together for sintering the powder, the first laser beam is brought to a small focus and the second to a large focus.